The present invention relates generally to a radio communication system and, more specifically, to techniques of control signal transmission in coordinated multi-point (CoMP) transmission/reception schemes.
Recently, LTE (Long Term Evolution)-Advanced standard has been developed for 4th generation system (4G), where the fairly aggressive target in system performance requirements have been defined, particularly in terms of spectrum efficiency for both downlink (DL) and uplink (UL) as indicated in the Sect. 8 of 3GPP TR 36.913 v9.0.0, Requirements for further advancements for Evolved Universal Terrestrial Radio Access (E-UTRA) (LTE-Advanced), December 2009 (hereinafter referred to as “NPL 1”). Considering the target of the cell-edge user throughput and the average cell throughput, which is set to be roughly much higher than that of LTE Release 8 (Rel. 8), multiple techniques, such as carrier aggregation, downlink enhanced MIMO, coordinated multi-point transmission/reception (CoMP), have been included in LTE-Advanced.
In Rel. 8/9/10, the downlink control channel (PDCCH) is defined to send control signal in Sect. 6.8 of 3GPP TS 36.211 v10.3.0, Physical Channels and Modulation for Evolved Universal Terrestrial Radio Access (E-UTRA) (Release 10) (hereinafter referred to as “NPL2”). Each UE's downlink control information (DCI) is aggregated into consecutive control channel elements (CCEs), where a control channel element corresponds to 9 RE groups as defined in Sect. 6.2.4 of NPL2. The DCI transports downlink or uplink scheduling information, requests for aperiodic CQI reports, notifications of uplink power control commands, etc. as described in the Sect. 5.3.3 of 3GPP TS 36.212 v10.3.0, Multiplexing and channel coding for Evolved Universal Terrestrial Radio Access (E-UTRA) (Release 10) (referred to as “NPL3”). The CCEs of multiple UEs connected to same serving cell are multiplexed and then scrambled by using a scrambling sequence initialized by a value cinit at the start of each subframe, which is a function of physical-layer cell identity (ID) of the serving cell as defined in the following equation in the Sect. 6.8.2 of NPL2 for interference randomization. In the following, the initialization value of scrambling sequence generation is called as the scrambling initialization value cinit for the sake of convenience.cinit=└ns/2┘29+NIDServCell  {Math. 1}where ns is the slot number within a radio frame.
The scrambled bit sequence is QPSK (Quadrature Phase Shift Keying)-modulated and mapped to the resource elements of PDCCH. The serving cell reserves a radio resource region for PDCCH of its UEs, i.e., whole bandwidth of first several OFDM symbols (max. 4 OFDM symbols) in a subframe. With the assistance of blind detection at UE side, only the location of the reserved radio resource region is required to be known by UE. The information of the location of the reserved radio resource is dynamically indicated by using L1/L2 signal through such as physical control format indicator channel (PCFICH), defined in the Sect. 6.7 of NPL2.
The present PDCCH, demodulated by cell-specific reference signal (CRS), is sent only by the serving cell and always occupies the entire system bandwidth of the first several OFDM symbols. It is not flexible to tailor the transmission characteristics of PDCCH to an individual UE and also impossible to coordinate transmissions in the frequency domain. This makes PDCCH ill-suited for new deployment, where the notion of a cell is less clear and where flexibility in how to transmit is needed to handle unexpected interference situations. Due to unexpected interferences, PDCCH capacity becomes a bottleneck when applying carrier aggregation, downlink enhanced MIMO and CoMP, etc.
In order to eliminate such a bottleneck, enhanced PDCCH (ePDCCH) has been proposed by R1-113155, Nokia (referred to as “NPL4”) and R1-113356, Ericsson, ST-Ericsson (referred to as “NPL5”). As shown in FIG. 1, the ePDCCH is sent over allocated resource blocks (RBs) in physical downlink data channel (PDSCH) area to increase the capacity and coverage of the control signal. The employment of UE-specific RS (DM-RS) in ePDCCH transmission makes the transmission properties transparent to the UE. In principle, the enhanced single-point MIMO as well as multi-point MIMO (i.e., CoMP) schemes for improving the throughput of data transmission becomes also available for the DL control signal transmission, as stated in NPL5. For the blind detection of ePDCCH at UE side, the location of the reserved radio resource region may be informed semi-statically (e.g., 120 ms, 240 ms, etc.) as the information element of E-PDCCH-Config by RRC signaling, similar to the way to inform the configuration of the relay PDCCH (R-PDCCH) as introduced in the Sect. 6.3.2 of 3GPP TS 36.331 v10.3.0, Radio resource control (RRC) and Protocol specification of Evolved Universal Terrestrial Radio Access (E-UTRA) (Release 10) (hereinafter referred to as “NPL6”).
For LTE-Advanced Rel. 11, CoMP has been agreed to be included as a tool to improve the coverage of high data rates, the cell-edge throughput, and also to increase system throughput as described in the Sect. 4 of 3GPP TR 36.819 v11.0.0, Coordinated multi-point operation for LTE physical layer aspects (Release 11) (hereinafter referred to as “NPL 7”). The CoMP schemes, joint transmission (JT), dynamic point selection (DPS), and coordinated scheduling/coordinated beamforming (CS/CB) are supposed to be supported as described in the Sect. 5.1.3 of NPL7. The CoMP cooperating set is defined in the Sect. 5.1.4 of NPL7 as a set of (geographically separated) points directly and/or indirectly participating in data transmission to a UE in time-frequency resource. In case of JT and DPS, UE's data, scrambled by a scrambling sequence with the serving cell's scrambling initialization value as defined in the Sect. 6.3.1 of NPL2, should be shared among more than one point in CoMP cooperating set; while, in case of CS/CB, data for a UE is only available at and transmitted from the one point (serving point) but user scheduling/beamforming decisions are made with coordinated among points corresponding to the CoMP cooperating set. It should be noted that the term “point” for coordinated multi-point transmission/reception can be used as a radio station, a transmission/reception unit, remote radio equipment (RRE) or distributed antenna of a base station, Node-B or eNB. Accordingly, hereinafter, a point, a radio station, a transmission/reception unit and a cell may be used synonymously.
According to the performance evaluation results in Sect. 7 of NPL7, JT/DPS CoMP achieves better performance than CB/CS to improve the cell-edge user throughput of downlink data transmission. For a cell-edge UE, which suffers from poor channel condition of serving point and strong interference from CoMP point, JT/DPS CoMP can also be applied to improve the capacity of its control signal in a similar way as that of data, by sharing not only data but also control signal, scrambled by a scrambling sequence with the serving cell's scrambling initialization value cinit among the selected transmission points (TPs).
A simple example of the above-described scheme is given in FIGS. 2A and 2B. Assuming that UE1 and UE2 have Cell1 as serving cell and Cell2 as CoMP cell as shown in FIG. 2A, ePDCCH can aggregate control information of the UE1 and UE2 using the same scrambling sequence for Cell1 and Cell2 as shown in FIG. 2B. As described in Section 6.8.2 of the NPL2, the scrambling sequence generation is initialized with the following initialization value cinit determined by the ID of Cell1 (serving cell).cinit=└ns/2┘29+NIDCell1  {Math. 2}
In the case of the UE2 with a different serving cell, however, the aggregation of control signal with CoMP cannot be made because different scrambling initialization values and different radio resources are used for the control signals of the UE1 and UE2, respectively. As shown in FIG. 3A, it is assumed that UE1 and UE2 are selected as CoMP UEs with multiple cooperating cells and the UE1 has Cell1 as serving cell and Cell2 102 as CoMP cell; while, the UE2 has Cell2 as the serving cell and Cell1 as the CoMP cell. For the employment of JT/DPS CoMP, the control signal of UE1, scrambled by using the Cell1's scrambling initialization value, is shared by Cell2. On the other hand, the control signal of UE2, scrambled by using the Cell2's scrambling initialization value, is shared by Cell1. Accordingly, the scrambling sequence generation is initialized with different initialization values cinit1 and cinit2 for Cell1 and Cell2, respectively:cinit1=└ns/2┘29+NIDCell1 cinit2=└ns/2┘29+NIDCell2  {Math. 3}
Besides their different scrambling initialization values, different radio resource regions are reserved at Cell1 and Cell2 for sending UE1's and UE2's control signals, respectively as shown in FIG. 3B. Within the previously reserved radio resource region, the occupied resource is dynamically allocated, resulting in remained resource.
In FIG. 3B, as an example, separate resources with max 3RBs for each one are reserved for each UE, but average 2RBs are used for each UE's control signal. As a consequence, an increasing number of CoMP UEs with different serving cells results in larger reserved radio resource regions in multiple cooperating cells.
An object of the present invention is to provide a method and system which can efficiently send control signals with improved capacity and coverage of a control signal for UEs with different serving cells.